Applicant's U.S. Pat. No. 4,928,030 teaches two- and three-axis piezoelectric actuators that position an object such as a rod or motor shaft by walking traction. A lifter piezoelectric actuator portion positions the actuator's traction member perpendicular to the object's surface. A tangenter piezoelectric actuator portion positions the actuator's traction member tangential to the object's surface. Lifter and tangenter portions of an actuator are integrally constructed and independently electrically controllable. A walking cycle consists of activating the lifter to apply a predetermined normal force between the traction member and the object while the tangenter translates the traction member at a speed equal to the surface speed of the object. During application and removal of normal force, no mechanical work is done by the traction member on the object. As the normal force is applied, a tangential strain is added to the tangenter portion. The product of the tangential force and the tangential distance traveled during the power stroke portion is the work done on the object. The work done per unit time, averaged over a complete cycle, is the power transmitted to the object.
At the end of the power portion of the cycle the tangential strain is removed as the normal force is removed by the lifter, still maintaining zero relative speed between object and traction member. As the traction member leaves the object's surface, the traction member retraces, that is, it reverses tangential stroke direction and changes speed until the opposite extreme tangential position is reached, thereby preparing for a new stroke. This is a smooth walking cycle because sliding is avoided. A pair of actuators alternately executes walking cycles, one actuator performing a power stroke while the other retraces. A predetermined coordinated positioning of the traction members of both actuators results in smooth walking. Smooth walking is defined as uninterrupted and smooth tractional power transmission without sliding.
The piezoelectric materials are generally electrically polarized ferroelectric ceramics. This class of materials is relatively brittle, having relatively little tensile strength. In addition, the temperature above the usual room temperature at which electrical polarization is irreversibly lost, usually called the Curie temperature, is relatively low. These physical properties are a detriment in some applications of walking actuators. U.S. Pat. No. 4,928,030 also teaches the use of relatively high applied voltages to achieve desirably large mechanical strokes. High voltages are a disadvantage in the context of solid state electronic drive devices, such devices having evinced more efficient operation with low voltages with relatively large currents.
Applicant's copending application Ser. No. 07/488,548 teaches the use of Fourier generation of nonsinusoidal mechanical wave forms needed for smooth walking. The teachings are primarily directed toward piezoelectric actuators, but are also directed toward electromagnetic actuators that function in a manner similar to piezoelectric ones. The benefits taught are relatively high electrical efficiency derived from resonant excitation of actuator portions, and relatively high electrical stability not normally associated with power amplifiers that drive preponderantly reactive electrical loads.